Timing attachment for a relay

ABSTRACT

A timing attachment for a relay including a light source and detector and an apertured control member movable between the light source and detector to influence transmission of light therebetween, the movement of the control member being controlled by the relay, and the light source and detector providing initiation of the timing. Reversal of the timing attachment on the relay changes the position of the attachment relative to the control member to enable change from time delay after energization of the relay to time delay after de-energization or vice versa.

United States Patent 1191 Clarke TIMING ATTACHMENT FOR A RELAY [75] Inventor: Geoffrey Clarke, Blunsdon, Nr.

Swindon, England [73] Assignee: Square D Company, Park Ridge, 111. 221 Filed: Mar. 16,1973 [21] Appl. No.: 342,327

[30] Foreign Application Priority Data Mar. 21, 1972 Great Britain 13107/72 52 11s. c1 317/130, 317/141 s [51] 1111. c1. 110111 47/18, I-lOlh 47/24 [58] Field of. Search 317/141 S, 130

[56] References Cited UNITED STATES PATENTS 8/1972 Boley et a1 317/141 s 1111 3,835,352 [451 Sept. 10, 1974 Primary Examiner-L. T. l-lix Attorney, Agent, or Firml-larold J. Rathbun [57] ABSTRACT A timing attachment for a relay including a light source and detector and an apertured control member movable between the light source and detector to influence transmission of light therebetween, the movement of the control member being controlled by the relay, and the light source and detector providing initiation of the timing. Reversal of the timing attachment on the relay changes the position of the attachment relative to the control member to enable change from time delay after energization of the relay to time delay after de-energization or vice versa.

8 Claims, 17 Drawing Figures PAIENIED SEP 1 01974 sum 2 0r 5 PATEN TED SEP 1 01974 sum 3 or 5 This invention relates to timing attachments for relays.

Pneumatic timing devices generally, provide good repeat accuracy with stable ambient temperatures. However, the temperature changes experienced in many environments ranging from cold on a winters day, through to an enclosure containing heat generating components mounted in direct sunlight, can cause timing variations of the order of percent (30C change). With suitable compensation, solid state timing variation would be less than 2 percent for the same temperature change. Failures of solid state timers are generally due to installation testing or abuse e.g. high potential tests, megger and continuity tests and momentary short-circuits. Electro-mechanical timers will normally survive these conditions.

Solid state timers are usually initiated by one of the following:

1. Application of power to the device serving dual roles of powering and initiating.

2. Using contacts to make or break an internal circuit of the timing relay.

3. Removal or application of a relatively high voltage to signal the initiation.

Method (1) is a good solution but cannot be used where normally closed (N/C) contacts or time delay after de-energization (TDD) functions are required.

Method (2) has several undesirable features e. g. poor noise immunity usually needs an isolated contact contact operates at low voltages unsuitable for most industrial relay contacts device can easily be destroyed by meggering, continuity checking or accidental application of a high voltage.

Method (3) overcomes most of the undesirable features of method (2) but requires additional circuitry in the form of filtering and potential dividing. Also it may have a limited operating voltage range.

According to the present invention there is provided a timing attachment for a relay, said attachment including a housing, anapertured control member adapted to be mounted on a movable contact carrier of the relay so as to be movable in the housing, a source of electromagnetic radiation and a detector so arranged in the housing that the detector can detect radiation transmitted from the source and transmission of the radiation is influenced by movement of the control member, a control circuit located in the housing, and an output contact electrically connectible with the control circuit, the detector being connected in the control circuit to control the operation of the output contact, and the housing being mountable on the relay in selected positions to allow for time delay after energization (TDE) or time delay after deenergization (TDD) of the relay.

According to the present invention there is also provided a timing attachment for a relay, said attachment including a housing, adapted to be mounted on the relay, a control circuit located in said housing, and an output contact removably mounted on said housing, the output contact being electrically connected with the control circuit which provides for time delay after energization of the timing attachment.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a top view of a first embodiment of a timing attachment;

FIG. 2 is a front view of the attachment of FIG. 1;

FIG. 3 is a sectional view on III-III of FIG. 2;

FIG. 4 is a bottom view of the attachment of FIG. 1;

the invention;

FIG. 8 is a sectional view on VIII-VIII of FIG. 7;

FIG. 9 is a sectional view on IX-IX of FIG. 7;

FIGS. 10a to 100 are perspective views of triac modules for use respectively with a timing attachment according to the invention, a solid state relay, and an all insulated device, illustrating how connections are made by means of mounting means;

FIG. 11 is a circuit diagram of the solid state control circuit of the timing attachment; and

FIG. 12 is a circuit diagram of a more simplified timing attachment.

Referring to FIGS. 1 to 6a of the drawings, a timing attachment 10 for a relay (not shown) provides selective time delay after energization (TDE) or time delay after deenergization (TDD) timing, and a normally open (N/O) or normally closed (N/C) output contact.

The attachment 10 includes a top housing 12, a bottom housing 14, and an output contact 16. The top housing 12 is mounted on the bottom housing 14 and defines therewith a compartment 18 wherein there are located a pair of printed circuit boards 20, 22 providing the solid state circuit shown in FIG. 11. The bottom housing 14 is arranged to fit on to contact blocks of a parent relay in a way similar to the pneumatic timing attachment of copending British Provisional Application No. 36977/7l (related to U.S. Pat. No. 3,775,710 issued on Nov. 27, 1973) or the latching attachment of copending British Provisional Application No. 53731/71 (related to U.S. patent application Ser. No. 307,746 filed on Nov. 17, 1972). A legend plate 24 is releasably secured to the upper face 26 of the top housing 12 by screws 28 for a purpose hereinafter described.

The bottom housing 14 (FIG. 5) has a pair of mounting lugs 30 depending one from each of two opposite sides 32 of a rectangular base member 34 for connection to the relay contact blocks, and an integral section 36 located centrally of, and extending upwardly from, an upper face 38 of the base member 34. A transverse slot 40 extends throughout the combined height of the base member 34 and the raised section 36, and a vane 42 is movable in the slot 40, the vane 42 being fastened in a recess on the top of a moving contact carrier 44 of the parent relay so as to slide in the slot 40 as the parent relay is energized or de-energized.

Recesses 46 are provided at each end of the base member 34 to seat terminals 48 and 50 (FIG. 4 which are connected to the respective circuit boards 22 and 20 (FIG. 3).

The vane 42 has two diagonally spaced transverse apertures 52 (FIGS. 6a to 6d). A source 54 of electromagnetic radiation in the form of an infra-red light emitting diode, and a detector 56 in the form of a phototachment is set up, ie the TDD or TDE function as hereinafter described.

The output contact 16 is in the form of a triac module (FIGS. 7 to 9) whichis relatively immune to damage except for overcurrent. Before the triac module is mounted on the attachment 10, terminals 59 are bent upwardly to extend substantially at right angles from an outer face 60 of the output contact 16. In this position the terminals 59 lie on the terminals 50 for electrical connection therewith when the contact 16 is mounted on the attachment 10. The means for mounting the contact 16 comprises a steel screw 62 (FIG. a) which passes through an aperture 63 (FIG. 8) provided in the module and a metal insert 64 rivetted to the circuit board 20 (FIG. 3). The insert 64 engages against a contact face 65 on the triac to electrically connect the gate of the triac with the solid state circuit. Alternatively, the output contact 16 can be in the form of a transistor.

The terminals 48 and 50 (FIG. 4) are each provided with a screw 66 (FIG. 3) engageable in an aperture 68 (FIG. 8) of the respective terminal, and a clamp 70 for making an electrical connection with other components.

, The triac module is replaceable and since it represents only percent of the total cost of the attachment it can be regarded as an expendable item. The output contact 16 is isolated from the input power supply an can switch a separate 110V supply. 1

It is evident that other devices eg proximity switches, counters, solid state relays, logic systems, etc. could use the same triac module. The problem is how to package and make connections to the triac. Some solutions are shown in FIGS.'l0b and 10p. These avoid the use of plugs and sockets, and again utilize mounting means in the form of a steel screw 72 (FIG. 10b), for making connection, or as shown, in FIG. 10c, utilize a nylon screw 74 and spring contact 76 to provide a totally insulated unit.

The timing attachment 10 is provided with a plug 78 (FIG. 1) which may be connected in two alternative positions in the circuit of the attachment (as can be seen in FIG. 11) to provide two different connections and accordingly give alternative modes of output contact operation. The legend plate 24 has a slot 80 at each of two opposite sides and the plug 78 projects through one of the slots 80, depending on the orientation of the legend plate 24 on the top housing 12. The plug 78 is secured against accidental removal by the plate 24 and the latter indicates, by the notation adjacent to the plug 78,'the mode of operation for which the attachment 10 is set up.

The operation of the timing attachment will now be described with reference to FIG. 11. Time Delay after Energization, Normally Open Timed Closed Contact (TDE, N.O.T.C.)

a. Reset Condition:

Power is continuously applied to terminals 48, the reactance of capacitor C3 acts as a voltage dropper or ballast, and the remaining alternating voltage is rectified by diodes Dl-D4 and filtered by capacitor C5.

The DC. voltage passes through a resistor R16 and the infra-red light emitting diode LED 1 which is protected against reverse voltage by a diode D5 in parallel. The DC. voltage is regulated by a zener diode 2D]. A capacitor C8 provides additional smoothing for the regulated supply. With the parent relay de-energized, light from LED 1 passes through an aligned vane aperture 52 (FIG. 6a) to the photo-transistor 1 PT. The phototransistor 1 PT conducts, and via a resistor R18 causes a transistor 5 TI to conduct which via a resistor R19 causes a transistor 6 TI to conduct. A capacitor C4, the timing capacitor, is rapidly charged to the reset conditionvia transistor 6 TI and a resistor R4. When transistor 6 TI conducts, current flows via a resistor R22 and causes a transistor 1 TI to conduct,which removes the base drive via a resistor R10 to a transistor 2 TI which is then cut-off. Current through a resistor R9 is then diverted via a resistor R14 to the base of a transistor 4 TI and via the NOTC selection link (plug 78) and aresistor R11 to the base of a transistor 3 TI. The transistor 3 TI conducts and holds the emitter of a unijunction transistor 1 UJT below its peak point voltage preventing it from firing the triac 1 TR which is in the reset condition. b. Timing and Timed Out Condition: Timing is initiated by energizing'the parent relay. This lowers the vane 42 and prevents lightfrom the light emitting diode LED 1- from reaching the phototransistor 1 PT (FIG. 6b). The phototransistor 1 PI, and. the transistors 5 TI and 6 TI are cutoff and the capacitor C4 begins to discharge, timing through two paths, (1) resistor R23-and potentiometer P 1 and (2) resistors R4, R22, the base-emitter junction of transistor 1 TI and resistor R6. When the voltage across the capacitor C4 falls below that required to provide sufficient current to keep the transistor 1 TI conducting, the transistor 1 TI begins to cut-off and its collector voltage slowly' rises. When currentstarts to flow into the transistor 2 TI base emitter junction regeneration occurs due to transistors 1 TI and .2 TI common emitter resistor R6, and transistor 2 TI rapidly switches into conduction. Resistor R9 is chosen to have a resistance considerably less than resistor R8. Thus when the transistor 2 TI conducts, the potential at the transistor 1 TI and the transistor 2 TI emitters will increase, effectively cutting off the discharge current of the capacitor C4 into transistor 1 TI base, resulting in a clean switching action between the transistors 1 TI and 2 TI. The potential divider, resistors R7 and R3, holds the emitters of the transistors 3 TI and 4 TI above the potential of the transistor 2 TI emitter. Therefore at the end of the timing period with the transistor 2 TI conducting, transistors 3 TI and 4 TI base-emitter junctions are reverse biased and cut-off. The emitter of the unijunction 1 UJT is now free to rise to its peak point voltage and fire the triac 1 TR. Time Delay after Energization, Normally Closed Timed Open Contact (TDE, N.C.T.O.)

The reset and timing conditions for this function are .identical to the TDE N.O.T.C. function except that the collector of the transistor 4 TI is connected to the resistor R11 via the NOTC link (plug 78). When the timer is in the reset condition, transistor 4 TI is conducting and transistor 3 TI is cut-off allowing the unijunction l UJT circuit to oscillate and fire the triac 1 TR.

At the end of the timing period, the transistor 4 TI is cut-off and the transistor 3 TI conducts 1 TR. the emitter of the unijunction l UJT below the peak point voltage and preventing it from firing the triac lTR.

Time Delay after De-energization, Normally Closed, Timed Closed Contact (TDD, N.C.T.C.)

To obtain the TDD function the timing attachment is removed from the relay contact block, turned through 180 and refitted to the relay. The vane 42 is left in situ, on the relay contact carrier 44 (to which it is keyed for direction), so the relative positions of vane 42 and timing attachment 10 will have changed.

a. Reset Condition:

At the instant when power is applied to the parent relay coil and the timer supply terminals 48, the parent relay will momentarily be in the timing or timed out state and light from the light emitting diode LED 1 will be blocked by the vane 42. Capacitor C4 will be in the timed-out state, transistor 1 TI cut-off, transistor 2 TI conducting and transistor 3 TI and 4 TI cut-off. The unijunction circuit therefore oscillates and fires the triac 1 TR. This is equivalent to the N.C. momentary contact which occurs on a TDD-NCTC contact of an electro-mechanical relay. The parent relay, now moves to its energized position and the vane 42 allows the light from the light emitting diode LED 1 to illuminate the phototransistor 1 PT (FIG. 6d) causing the transistors 5 TI and 6 TI to conduct and the capacitor C4 to reset. When the transistor 6 TI conducts, the transistor 1 TI conducts, the transistor 2 TI is cut-off and the transistor 3 T1 conducts stopping the unijunction 1 UJ T circuit from oscillating and firing the triac 1 TR.

b. Timing and Timed Out Conditions:

When the parent relay is tie-energized the vane 42 moves to block the light from the light emitting diode LED 1 to the phototransistor 1 PT (FIG. 6c), and the V transistors 5 TI and 6 TI are cut-off. Capacitor C4 then discharges. When the voltage across the capacitor C4 is too low to keep the transistor 1 TI conducting, the transistor 1 TI cuts off and the transistor 2 TI conducts cutting off the transistors 3 TI and 4 TI thus allowing the unijunction l UJT circuit to oscillate and fire. the triac 1 TR.

Time Delay after De-energization, Normally Open, Timed Open Contact (TDD, N.O.T.O.)

a. Reset Condition:

At the instant when power is applied to the parent relay coil and the timer 10, the initiating relay will momentarily be in the timing or timed out state and light from the light emitting diode LED 1 will be blocked by the vane 42. Capacitor C4 will. be in timed-out state and the transistor 1 TI will be cut-off, the transistor 2 Tl conducting and transistor 4 TI cut-off. The collector of the transistor 4 TI is linked to the resistor R11 by the NCTO-NOTO link (plug 78) and the transistor 3T1 is therefore conducting and preventing the unijunction 1 UJT circuit from oscillating and firing the triac 1 TR. This state is equivalent to the momentarily open condition of a NOTO contact on a TDD electro-mechanical relay.

The parent relay now moves to its energized position and the vane 42 allows the light from the light emitting diode LED 1 to illuminate the phototransistor 1 PT causing the transistors 5 Tl and 6 TI to conduct and the capacitor C4 to reset. When the transistor 6 TI conducts, the transistor 1 TI conducts, the transistor 2 Ti is cut-off, the transistor 4 Tl conducts and, via the NOTO link (plug 78), 3 TI is cut-off, allowing the unijunction 1 UJT circuit to oscillate and fire the triac 1 TR.

b. Timing and Timed Out Condition:

When the parent relay is de-energized the vane 42 moves to block the light from the light emitting diode LED 1 to the phototransistor 1 PT and the transistors 5 TI and 6 TI are cut-off. Capacitor C4 the transistors discharges. When the voltage across the capacitor C4 is too low to keep the transistor 1 TI conducting, the transistor 1 TI cuts off, and the transistor 2 TI conducts cutting off the transistor 4 Ti. The transistor 3 TI conducts, stopping the unijunction 1 UJT circuit from oscillating and firing the triac 1 TR.

In the development stage of the timing attachment 10, a problem was encountered when the supply to the timer was switched off, with the timer, set up to perform TDD, NOTO and TDE NCTO functions, in the timed out state. When the 24V supply has decayed to about 8V, the circuit comprising transistors 1 TI and 2 TI switched back to its reset state and the unijunction l UJT circuit fired the triac 1 TR until the positive supply decayed to a very low value. To overcome this, a 10V zener diode ZD2 is included in the base B2 circuit of the unijunction transistor 1 UJT so as to prevent it from oscillating below a supply voltage of 10V.

The variation of time delay with temperature for a temperature change of 40C was 10 percent for the original timing circuit used in this development. To reduce this variation to less than 2 percent, a temperature compensation circuit consisting of resistors R15, R13, thermistors THl, TH2 and diode D6 is provided in the circuit. Therrnistors Tl-Il and TI-l2 are negative temperature co-efficient thermistors which together with resistors R15, R13 and diode D6 have the effect of reducing the voltage to which capacitor C4 can charge, when reset, as the temperature increases. This reduction in the charged up voltage of the capacitor C4 compensates within 2 percent for the increase in time delay due to timing circuit component variations with temperature.

The advantages of using this technique are that it eliminates the problems associated with method (2). The attachment does not impose a heavy mechanical load on the operating relay and it provides for selection of TDE and TDD timing functions.

FIG. 12 shows a solid state circuit for use in a simplified timing attachment which provides only a TDE timing function with a normally open timed closed output contact. With this attachment the light emitting diode and the photo transistor are omitted and the vane is not fitted to the relay contact carrier. Also the reversible plug is not included in the timing attachment'assembly. The device uses the initiation method (1) where the application of power to the timer terminals serves the dual role of powering the device and initiating the timing function.

The output contact again comprises a triac TRl and is as shown in FIGS. 7 to 9 and as hereinbefore described.

The operation of this timing attachment is as follows:

a. Timing and Timed Out Conditions: Power is applied to the input terminals, a resistor R1 acts as a voltage dropper, and the remaining alternating voltage is rectified by diodes D1 D4 and filtered by a capacitor C1 .The' DC voltage passes through resistor R2 and is regulated by zener' diodes ZDl and ZD2. A timing capacitor C3 is initially in the discharged state, a transistor 2 TI is cutoff and a transistor 3 TI is conducting, holding the emitter of a unijunction transistor 1 UJT below its peak point voltage and preventing it from oscillating and firing a triac TRl.

The reset transistor 1 TI is reverse biased and cut-off under this condition.

Capacitor C3 will now charge uptowards the voltage set by a resistor R6 and a potentiometer P1. When the voltage across the capacitor C3is sufficient to cause the transistor 2 TI to conduct and the transistor 3 TI to be cut-off the unijunctiontransistor 1 UJT circuit oscillates and fires the triac TRl. The switching .circuit is formed by resistors R8, R9, R10, R11 and transistors 2Ti and 3 TI and is designed to give a rapid switching action when the capacitor C3 charges up to a certain voltage.

b. Reset Condition:

When the power is removed from the input terminals,

capacitor C1 starts to discharge.

Capacitor C2 conveys the change in potential of the positive supply line and forward biases the base of the transistor 1 T1, which conducts and discharges the capacitor C3 via the resistor R3. This reset circuit gives the timer a very fast reset timeaCapacitors C8 and C9 overcome noise problems.

As soon as the voltage across the capacitor C3 has fallen below that required to keep the transistor 2 TI in conduction, the transistor 3 TI switches to the conducting condition. The emitter of the unijunction transistor -1 UJT is now held below its peak point voltage and the unijunction transistor 1 UJT circuit stops oscillating and firing the triac TRl. p

A timing attachment as hereinbefore described can obtain good timing repeat accuracy with temperature, by using solid state techniques and is generally abuseproof or is provided with low cost readily replaceable parts.

What we claim is:

1. A timing attachment for an electromagnetic relay having a movable contact carrier, said timing attachment comprising a housing a source of radiation and a radiation detector mounted in spaced apart relationship in the housing so that the detector can detent radiation transmitted from the source to the detector along a path, means for mounting the housing on the relay, a control member having a pair of apertures and movable in the housing in intersecting relation with the path when the housing is mounted on the relaythereby to influence the radiation transmitted from the source to the detector, means for mounting the control member on the contactcarrier for movement thereby, a control cicuit inthe housing, a contact interposed in the control circuit, means causingoperation of the contact in response to the amount of radiation received. by the detector along the path, means for reversing the relative position of the control member with respect to the path so that in one position a first portion of the control member isin the path and in the other position a second portion of the control member is in the path, said apertures beingin said portions, respectively, and positioned so that in one position of the member with respect to the path, a firstone of the pair of apertures is in the path when the relay is deenergized and the path is blocked by the control member when the relay is energized and, in the other position of the member with respect to the path, a second one of the pair of apertures is in the path when the relay is energized and the path is blocked by'the control member when the relay .is'deenergized.

*2. A timing attachment as in claim 1 wherein the means for mounting the housing on the relay permits the housing to be mounted on the relay selectively in two positions in one of which the member is in one of its two positions with respect to the path and in the other the member is in the other of its two positions.

3. A timing attachment as claimed in claim 1, wherein the apertured control member comprises a generally rectangular vane and the apertures are spaced generally along a diagonal of the vane.

4. A timing attachment as in claim 1, wherein the source of radiation is a light emitting diode.

5. A timing attachment as in claim 1, wherein the detector is a photo transistor.

6. A timing attachment as in claim 5, wherein the output contact comprises a triac, and means are provided for removably mounting the output contact on the housing, the mounting means forming an electrical connection between the triac and the control circuit.

7. A timing attachment as in claim 5, wherein the output contact comprises a transistor, and means are provided for removably mounting the output contact on the housing, the mounting means forming an electrical contact between the transistor and the control circuit.

8. A timing attachment as in claim 1 wherein the control circuit incorporates a transistor, the conduction or non-conduction of which controls the operation of the output contact, and wherein there is provided a plug member which is arranged to be mounted in the housing in either of two locations to provide different ways of connecting the transistor in the control circuit and accordingly provide selected modes of output contact operation, and a legend plate having notations at various locations, to indicate, by the notations adjacent to the plug member, the selected mode of operation.

mg? I UNITED- STATES PATENT OFFICE.

CERTIFICATE OF CORRECTION Patent No- 3.85.252 @D 't Sentember 10.197 4 ltivent fl Geo ffrev' Clarke It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column l, line 33, change -"1PI"'to' --'lPT-. Column 5, line 4, Change 5Q. .jhQllll ii..liQ lllfi liulfll firxmlQ hqQ lqel. ".'..t..h trans'istor's", ,and insert then Column 7, li -F 9, change T 5- I Signed and Sealed tbi s 7th .oIf' Jahfiary 19.75.

McCOYM. IGIBSONJRY. Q -C.':YMARSHALLDA'NNV Co' jrmni ss'ioner of Patents Atte'e-tfing- Off-i'c er 22;;35. UNiTED' STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. $835,252 Dated SeDtember 10. 1914 ltivent fl Geoffrev Clarke It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

Column l, line 33, change "1PI"'t 0' -lPT--. Column 5, line 4, Change 0. -:h. l. il,n ::..,;;;Q9.l..@@.;.;l ne:. lQ; Qn.,c4.elVI'th transistors ,and insert then Y 1 Col 7, i i eijlg change |2Ti"' t 2T1 "-1".

. 'Sig nedand sealed thi s' 7th of January 1975.

.(SEALY .Atte's'tgr Mc'coYfi. GIBSON "JR. Y CQMARSHALL DA'NN Atte'gtingOffi'er I Cqimni -ssioner bf Patents 

1. A timing attachment for an electromagnetic relay having a movable contact carrier, said timing attachment comprising a housing a source of radiation and a radiation detector mounted in spaced apart relationship in the housing so that the detector can detent radiation transmitted from the source to the detector along a path, means for mounting the housing on the relay, a control member having a pair of apertures and movable in the housing in intersecting relation with the path when the housing is mounted on the relay thereby to influence the radiation transmitted from the source to the detector, means for mounting the control member on the contact carrier for movement thereby, a control cicuit in the housing, a contact interposed in the control circuit, means causing operation of the contact in response to the amount of radiation received by the detector along the path, means for reversing the relative position of the control member with respect to the path so that in one position a first portion of the control member is in the path and in the other position a second portion of the control member is in the path, said apertures being in said portions, respectively, and positioned so that in one position of the member with respect to the path, a first one of the pair of apertures is in the path when the relay is deenergized and the path is blocked by the control member when the relay is energized and, in the other position of the member with respect to the path, a second one of the pair of apertures is in the path when the relay is energized and the path is blocked by the control member when the relay is deenergized.
 2. A timing attachment as in claim 1 wherein the means for mounting the housing on the relay permits the housing to be mounted on the relay selectively in two positions in one of which the member is in one of its two positions with respect to the path and in the other the member is in the other of its two positions.
 3. A timing attachment as claimed in claim 1, wherein the apertured control member comprises a generally rectangular vane and the apertures are spaced generally along a diagonal of the vane.
 4. A timing attachment as in claim 1, wherein the source of radiation is a light emitting diode.
 5. A timing attachment as in claim 1, wherein the detector is a photo transistor.
 6. A timing attachment as in claim 5, wherein the output contact comprises a triac, and means are provided for removably mounting the output contact on the housing, the mounting means forming an electrical connection between the triac and the control circuit.
 7. A timing attachment as in claim 5, wherein the output contact comprises a transistor, and means are provided for removably mounting the output contact on the housing, the mounting means forming an electrical contact between the transistor and the control circuit.
 8. A timing attachment as in claim 1 wherein the control circuit incorporates a transistor, the conduction or non-conduction of which controls the operation of the output contact, and wherein there is provided a plug member which is arranged to be mounted in the housing in either of two locations to provide different ways of connecting the transistor in the control circuit and accordingly provide selected modes of output contact operation, and a legend plate having notations at various locations, to indicate, by the notations adjacent to the plug member, the selected mode of operation. 